The spatial dose distribution across a feature, derived by a conventional 2-D proximity effect correction scheme, is a "V-shape," i.e., higher dose closer to edges. With such a dose distribution, it is extremely difficult to realize a vertical sidewall in the resist profile for nanoscale features while reducing the critical dimension (CD) error. In this paper, it is shown that, in order to achieve a vertical sidewall of nanoscale feature with the minimum total dose, a dose distribution of a shape other than the V-shape must be used. This is due to the fact that the lateral development of resist becomes comparable to the vertical development for nanoscale features and the exposure varies along the depth dimension with high and low contrasts at the top and bottom layers of resist, respectively. Based on these characteristics, new types of dose distributions, i.e., "M-shape" and "A-shape," have been derived to achieve a target resist profile of a vertical sidewall while minimizing the total dose. The simulation results show that the new dose distribution types lead to a more vertical sidewall, a smaller CD error, and a lower total dose. V
One of the major limiting factors in electron beam (e-beam) lithography is the geometric distortion of written features due to electron scattering, which is known as the proximity effect. A conventional approach to the proximity effect correction (PEC) is, through 2D simulation, to determine the dose distribution and/or shape modification for each feature in a circuit pattern such that the written pattern is as close to the target pattern as possible. Earlier, it was shown that the 3D PEC, which considers the variation of exposure along the resist-depth dimension, would be necessary for the feature size well below 100 nm. Also, a feature-by-feature correction procedure is too time-consuming to be practical, especially for the 3D PEC of large-scale patterns. In this paper, a new method for the 3D PEC is proposed, which adopts 3D resist profile (instead of 2D exposure distribution) in optimization, but avoids the intensive computation by employing a critical-location-based correction procedure. The proposed method achieves 3D resist profiles closer to the target ones, compared to 2D PEC. The simulation results show that the proposed method has a potential to provide a practical and effective alternative to the conventional approach.
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